1. Field of the Invention
The present invention relates to protector modules which are used in various locations such as telephone central offices to protect the inside equipment from damage as a result of overvoltage and overcurrent conditions that may occur on the outside lines and more particularly to such a module which uses positive temperature coefficient resistors (PTCRs) to provide overcurrent protection.
2. Description of the Prior Art
There are many well known modules which are used at telephone company central offices and increasingly at other locations to provide protection against any overvoltage and overcurrent conditions that may occur on the telephone line pair. One example of such a module is that shown in U.S. Pat. No. 3,975,664 (hereinafter the '664 patent) which is assigned to the same assignee as is the present invention.
The module described in the '664 patent uses a three element cold cathode gas tube to provide protection against an overvoltage condition on the telephone line pair to which the module is connected. Protection against a sustained overcurrent condition (an overcurrent associated with an overvoltage of sufficient amplitude to cause the gas tube to conduct) on the line pair is provided by having slugs of low melting point solder in contact with the end terminals of the gas tube. The occurrence of a sustained overcurrent condition on either line of the line pair protected by the module causes the associated slug to melt. The melting of the slug allows the associated end terminal of the gas tube to come in contact with the ground terminal of the module, there by permanently grounding the associated line. A more detailed description of how the module shown in the '664 patent provides overvoltage and sustained overcurrent protection may be obtained by referring to that patent.
The module described in the '664 patent does not provide protection against a marginal overcurrent condition, i.e., an overcurrent condition associated with an overvoltage which is not of sufficient amplitude to cause the gas tube to conduct. In other words, a marginal overcurrent does not flow through the gas tube. Therefore, the module of the '664 patent should not be used with those line pairs where such a condition may occur.
As discussed in the '664 patent, the module disclosed therein has certain advantages. Among them are the elimination of a spring to carry large overcurrents of long duration, relatively short current paths, standard 5 pin base, ease of assembly and its relatively small and compact configuration. It is desirable that such a module also be capable of protecting against marginal overcurrents.
Protection against such overcurrents has typically been provided by using an in-line heat coil which heats a fusible solder element. The solder element melts during the occurrence of such a condition. One example of a protector module which uses heat coils is disclosed in U.S. Pat. No. 3,849,750 (hereinafter the '750 patent) also assigned to the same assignee as is the present invention. As shown in the '750 patent, the heat coil and overvoltage protection device, e.g., two element gas tube, are in-line and coaxial. In addition, a spring is used to bring a pair of contacts together to form the direct metallic path from the line to ground when the heat coil fusible element melts.
While it is desirable to provide protection against marginal overcurrents in a module of the type shown in the '664 patent, it is not feasible to use a heat coil therein for such protection. A comparison of the structure of the modules of the '664 and '750 patents makes that clear. In addition, a heat coil is undesirable in that when it does operate to provide protection against marginal overcurrents, the fusible solder melts and the line is permanently grounded. It is then necessary to replace the module. Until that is done, the telephone line is out of service.
It was then decided to provide such protection in a module of the type shown in the '664 patent by using a solid state device, such as a PTCR, whose resistance substantially increases when current flow through the device causes the device temperature to reach a predetermined temperature above ambient. Such a device performs the same function as a heat coil in that current is limited, but without the undesirable permanent grounding of the line described above. The flow of an overcurrent through the device causes its temperature to reach that temperature at which the resistance of the device abruptly increases by several orders of magnitude to thereby limit the current. When the overcurrent condition ceases, the device cools and its resistance decreases to its ambient temperature value. The device is then ready for the next occurrence of an overcurrent condition.